function unite(arr1, arr2, arr3) {
    let args = Array.from(arguments);
    let arr = args.reduce(function (prev, cur) {
        return prev.concat(cur);
    });
    return arr.filter(function (item, index, arr) {
        return arr.indexOf(item) === index;
    });
}


console.log(unite([1, 3, 2], [5, 2, 1, 4], [2, 1]));
